There are already known various constructions of fuel cells, among them such employing a proton exchange membrane confined between respective cathode and anode electrode plates. In such fuel cells, a gaseous fuel and an oxidizing gas are supplied to the anode electrode plate and to the cathodes electrode plate, respectively, and distributed as uniformly as possible over the active surfaces of the respective electrode plates (that is, the electrode plate surfaces facing the proton exchange membrane, each of which is usually provided with a layer of catalyst). Further, an electrochemical reaction takes place at and between such electrode plates, with attendant formation of a product of the reaction between the fuel and oxygen (product water), release of thermal energy, creation of an electrical potential difference between the electrode plates, with the thus generated electric power usually constituting the useful output of the fuel cell.
In the prior art proton exchange membrane fuel cells, each of the electrode plates typically has an associated electrically conductive backing plate having a relatively substantial thickness. Typically, each of these backing plates has a plurality of flow passages or grooves in the surface facing the electrode plate which carries the respective fuel or oxidant over the surface of the respective electrode, namely, the anode on the fuel side and the cathode on the oxidant side. Grooves in the backing plates also provide a means of supplying coolant over the central region of the unit cell. In conventional designs, the interior coolant channels are generally formed by the cooperating surfaces of two backing or separator plates, one of which contains grooves engraved, milled or molded in its surface and the other of which is planar. The coolant channels are located at periodic intervals along the stack. The backing plates are electrically coupled in series to provide a path for conducting electrons between electrodes.
Unit cells are thus constructed of the proton exchange membrane surrounded by electrodes which are further surrounded by the aforementioned grooved backing plates. Two or more unit fuel cells can be connected in series or in parallel to increase the overall power output of the assembly. In such arrangements the cells are typically connected in series such that the anode backing plate of one unit cell abuts the cathode backing plate of the adjacent unit cell. Such assemblies are referred to as fuel cell stacks. An example of a prior art fuel cell stack is disclosed in U.S. Pat. No. 5,252,410.
Unfortunately, the use of grooved backing plates leads to thick unit cells and low cell area densities. Cell area density (CAD) is defined as the square meters of active cell area per kilogram of stack weight. Cell area density is directly related to stack power density but is independent of operating condition. Accordingly, an object of the present invention is to increase the cell area density of a fuel cell stack.
The viability of prior art fuel cell stacks has also suffered due to performance issues involving inadequate removal of product water. Inadequate water removal leads to poor oxygen transport to active catalyst sites on the cathode which then lead to low cell voltage. Consequently, it is an additional objective of the present invention to provide a design which insures that excessive product water does not collect on the cathode.
It is yet another objective of the present invention to design the fuel cell stack of the above type in such a manner as to be relatively simple in construction, inexpensive to manufacture , easy to use, easy to maintain, and yet reliable to operate.